The present invention relates to the field of signal processing. More specifically, it pertains to a method and apparatus for performing echo cancellation that finds applications in digital mobile systems, such as CDMA wireless systems, among other possibilities.
A common issue within telecommunication networks is the echo that arises as a result of transmitted signals being coupled into a return path and fed back to the respective sources. The most common cause of the coupling is an impedance mismatch at a four-wire to two-wire hybrid. The impedance mismatches cause signals in the incoming branch of a four-wire circuit to get coupled into the outgoing branch and return to the source. A standard form of echo control is echo cancellation. Echo cancellers operate by modeling the echo path to subtract a properly delayed and attenuated copy of a transmitted signal from the receive signal to attenuate (cancel) echo components. Generally, echos are canceled close to the source so that delays in the echo canceller are minimized.
Telephone companies traditionally deploy echo cancellers only in long distance toll circuits. In the advent of digital mobile (digital cellular phone) technology, the typical round trip delay of such digital mobile systems is around 200 ms due to the digital processing delays of each element (e.g. speech encoding/decoding, data packetizing, etc) along the path. The length of the delay introduced by the digital mobile systems is comparable to that of a long distance call. Echo cancellers are therefore needed in the base stations of the digital mobile systems even for local (as opposed to long distance) call applications.
In general, traditional echo cancellers perform well in digital mobile systems; however, there is one important scenario that is unique to digital mobile systems and does not exist in traditional landline circuits. This scenario is the call characterized by a lack of hybrid along the entire signal path, such as in the case of a call from a digital mobile to a digital mobile or from a digital mobile to a digital PBX termination. Unfortunately, the traditional echo canceller designs do not perform well under no-hybrid conditions because the echo canceller is designed to cancel echo. Specifically, when there is: no hybrid and thus no echo being generated, traditional echo cancellers are generally known to cause spurious echo and unpleasant speech clipping over the communication channel.
The background information herein clearly shows that there exists a need in the industry to provide an improved method and apparatus for performing echo cancellation, particularly in situations where the signal path may be such that in some instances echo is present while in other instances echo is absent.
The present invention is directed to an echo cancellation device for performing echo cancellation on a voice signal that can acquire an inactive operative mode when a certain condition exists, the condition indicating that no audible echo is present in the voice signal. In a first example of implementation, the condition is the actual level of echo in the voice signal. When the level of echo is low or non-existent the echo cancellation device acquires the inactive operative mode and allows the propagation of the voice signal without performing any echo cancellation. In a second example of implementation the condition that reflects the level of echo is the type of connection between the terminals involved in the voice call. Some types of connections are inherently substantially echo-free. This is the case of a digital mobile to a digital mobile connection or a digital mobile to a digital PBX (Private Branch Exchange) termination connection. When the echo cancellation device determines that an echo-free connection is being established, it acquires the inactive operative mode.
Under the first example of implementation, the echo cancellation device receives both a near-end voice signal and a far-end voice signal from a channel in a communication system, and includes an echo cancellation unit and a control unit. The near-end voice signal is the potentially echo-corrupted voice signal while the far-end voice signal is the precursor of the echo in the near-end voice signal. The echo cancellation unit is responsible for processing the far-end voice signal to reduce a magnitude of an echo occurring in the near-end voice signal, in order to generate an echo-attenuated voice signal. The control unit is responsible for assessing a level of echo in the near-end voice signal and is responsive to a condition in which the level of echo in the near-end voice signal is below a certain threshold to cause the near-end voice signal to be released from the output of the echo cancellation device.
By setting the echo cancellation device in the inactive operative mode when echo is not present in the communication channel, the echo cancellation device will not attempt to cancel echo where there is none, the latter causing spurious echo and unpleasant speech clipping in the communication channel. Thus, the quality of speech transmission provided in the communication channel is greatly improved.
Throughout the specification, the term xe2x80x9chybridxe2x80x9d is used to refer to any physical mechanism causing electrically generated echo, such as an impedance mismatch between two wires.